1. Field of the Invention
The present invention relates to an imaging lens, an image reading apparatus and an image forming apparatus, which includes the imaging lens.
2. Description of Related Art
An image reading section or an image scanner for a facsimile or a digital copying machine digitalizes a draft image by reading the image information with an imaging lens and forming a reduced image on a solid photographing element, such as a CCD.
In order to read the draft image in full color, there has been an optical system which digitalizes a color image by using a 3-line CCD which has for example, light receiving elements with red, green and blue filters arranged in 3 lines in a chip to separate chromatically the draft image formed on a light receiving surface of the CCD into 3 elementary colors.
Generally, it is required for the imaging lens to have a high contrast at a high spatial frequency region in a field surface and a vignetting factor, in other words, aperture efficiency close to 100%.
Furthermore, in order to read the draft image in full color perfectly, a chromatic aberration correction must be performed correctly to align each imaging position with red, green or blue color on an imaging surface along an optical axis of the imaging lens.
Therefore, it is necessary to control a curvature of field of the imaging lens as far as possible in order to obtain an imaging quality uniform for an image with various image heights from a paraxial domain to the margins.
There has been disclosed conventionally a Gaussian lens having a 6-lens in 4-group configuration as an imaging lens in Japan Patent Laid-Open No. H6-109971, H10-68881, H10-253881 and H11-109221.
It is possible for the Gaussian imaging lens to correct the curvature of field within a field angle up to 20°, and even to inhibit a coma flare at a relatively greater lens diameter. However, a relatively greater lens diameter will inevitably cause a greater outer lens dimension, and accordingly, there is a limitation to miniaturize the Gaussian imaging lens or an apparatus applying the Gaussian imaging lens thereon and to reduce a manufacture cost, since the Gaussian imaging lens is made from lenses as many as 6 pieces.
Furthermore, it is difficult to assure an imaging quality of the Gaussian reading lens made from only spherical lenses when a reading resolution is for example as high as 600 dpi and a line sensor has a resolution power as small as for example 4.7 μm per pixel.
For a lens, an aberration of the curvature of field may vary according to a manufacture error on parameters such as a radius of curvature, a lens thickness and a lens interval etc., and as a result it is impossible to obtain a uniform imaging quality for an image with various image heights from the paraxial domain to the margins. In order to counteract the variation for each parameter so as to ensure the imaging quality, it is necessary to select and combine lenses with different thickness, or change spatial intervals between lenses.
Therefore, there has been disclosed in Japan Patent Laid-Open No. 2004-304686 a novel lens in which an aspherical lens is adopted in order to improve the imaging quality.
However, when there is a surface shape error to some extent occurred on a lens, it is impossible to be corrected simply by lens selection or lens interval adjustment. Herein the surface shape error is defined as a deviation between a surface shape of a lens and a proximal spherical surface thereof.